Solderable lead

ABSTRACT

A solderable lead is fashioned with a C-shaped contact portion along its length. The interior of the C-portion is dimensioned to receive and resiliently contact a substrate at a predetermined contact pad. A layer of solder is provided on the exterior of the C-shaped contact portion and extends around its edge. During heating, the solder migrates to the interior of the C-portion by capillary action to the area between the contact pad of the substrate and the interior of the C-portion. While the solder is liquid, the C-portion maintains engagement with the substrate.

BACKGROUND OF THE INVENTION

The present invention relates to solderable terminals and leads,particularly solder clips intended for connection to contact pads onedges of substrates.

Several leads exist in the market in the form of a clip adaptedresiliently to engage the edges of a substrate, while a mass of solderis held on the clip adjacent the position of a conductive area or pad onthe substrate. Upon heating the substrate/terminal assembly, the soldermass melts and is drawn, usually by capillary action, to the area ofcontact between the terminal and the substrate conductive area. Examplesof such solderable clips are shown in U.S. Pat Nos. 4,120,558;4,019,803; 4,203,648; 4,357,069; 4,367,910; 4,592,617; 4,605,278;4,697,865; 4,728,305; and 4,780,098. Unfortunately, these terminalsgenerally involve complex structures, such as curved fingers or tabs,for forming the lead and holding the solder mass in place until theassembly is heated, requiring specialized tooling and productiontechniques.

A device disclosed in U.S. Pat. No. 4,900,279 to Dennis partially solvesthis problem. This devices uses a basically C-shaped clip that is coatedon the interior of the C with solder. The substrate is inserted into theinterior of the C, bringing the contact pads of the substrate in contactwith the solder, which is then melted, forming when re-solidified amechanical and electrical connection between the clip and the substrate.Unfortunately, when the solder melts, the significant layer of liquidsolder between the clip and the substrate create an unstable hold on thesubstrate. The substrate tends to float or slide around in relation tothe clip during this period. This can potentially lead to a solderconnection of the clip to an area partially or completely off of itsdesired contact pad, which may cause a short-circuit to an adjacent,closely spaced contact pad.

SUMMARY OF THE INVENTION

Thus, it is an object of the invention to provide an improved simplesolderable terminal having an easily applied and relatively nearbysource of solder, while providing a firm grasp on a correspondingsubstrate throughout the soldering process.

It is a further object of the invention that the solder terminal beeasily and simply fabricated with a minimum of manufacturing steps.

In accordance with the invention, a solderable lead is fashioned with aC-shaped contact element formed along its length. The interior of theC-element is dimensioned to receive and resiliently engage a substrateat a predetermined contact point such as a contact pad on the substrate.A layer of solder is provided on the exterior side of the C-element.During heating, the solder migrates to the interior of the C-element bycapillary action to the area between the contact pad of the substrateand the C-element. Thus, while the solder is liquid, the resilientC-element holds the lead engaged with the substrate. To aid in themigration of the solder, the side edges of the layer of solder arecurved toward the interior of the C-element by simple expedients duringmanufacture.

The foregoing and other objects and advantages of this invention willbecome apparent to those skilled in the art upon reading the detaileddescription of a preferred embodiment in conjunction with a review ofthe appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a fragment of a conductive strip shoWing twosolderable leads according to the invention connected to a carrierstrip.

FIG. 2 is a side view of a solderable lead according to the invention,taken along the line 2--2 of FIG. 1.

FIG. 3 is a rear view of a fragment of a conductive strip duringmanufacture of solderable leads according to the invention.

FIG. 4 is a cross-section of the contact portion of the solderable lead,mounted on a substrate, viewed along line 4--4 of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show a preferred embodiment of the invention in the formof aseries of leads 10 each having a C-clip 12 for receiving andgrasping a substrate 50 (such as a printed circuit board, integratedcircuit, chip carrier or the like). The leads 10 are formed from acontinuous strip 11 of semi-resilient conductive material through aseries of steps, such as by progressive stamping, including punching andbending.

FIG. 3 shows a fragmentary portion of the strip 11 from which the leads10 are formed. The semi-resilient conductive material (such as berylliumcopper) has two bands of solder 28 secured thereto, such as by coldwelding the solder bands 28 to the conductive strip 11. Other methods ofapplying the solder bands 28 to the strip 11 may of course by used.

The strip 11 is punched at lines 15 to form the individual lead bodies17. Indexing holes 16 are also punched along the edge of strip 11 toform a carrier strip 14.

During manufacture, the leads 10 remain connected to the carrier strip14. The indexing holes 16 are used to move the leads 10 through amanufacturing apparatus such as a progressive stamping machine, and tomaintain the precise inter-lead spacing necessary for the leads 10 tomatewith respective similarly spaced contact pads 18 on the substrate50. Extending from the carrier strip 14 for each lead 10 is an elongatedbody portion 20 for eventual connection of the lead 10 to otherelectronic components.

Adjacent the end of the body portion 20 remote from the carrier 14 isthe C-clip 12. The C-clip 12 is formed of four generally right anglebends, the two outer bends being convex with respect to the front of thelead 10,while the two inner bends are concave. The bends may be formedat the linesmarked "a", "b", "c", and "d" on FIG. 3. The lengths of thetop and bottom walls 22,24 of the C-clip 12, as well as the length ofthe rear wall 26 are determined by the thickness of the substrate 50 onwhich the clip 12 is to be mounted and the positioning of the contactpoints 18 thereon.

As pointed out above, preferably before the leads 10 are punched fromthe sheet 11 of stock material, the bands of solder 28 are deposited onthe back side of the leads 10 in the area that is to be bent into theC-clip 12. Alternatively, the solder bands 28 may be formed in a singleband, adapted to cover not only the top and bottom walls 22,24 of theC-clip 12,but also its rear wall 26. Where connection is desired to onlyone contact pad 18 on one side of substrate 50, a corresponding one ofthe solder bands 28 may be omitted. The solder bands 28 are thick enoughto provide asufficient amount of solder to positively connect the C-clip12 to the contact pad 18 of the substrate 50 after heating.

According to a feature of the invention, the flow of solder to thecontact pad is facilitated by specially forming the solder band or bands28 on theindividual lead 10. Thus, when the lead 10 is punched from thesheet 11, a portion of solder 30 is caused to extend laterally beyondthe width of thebody 20 and is curved toward the interior of the C-clip12, to cover not only the exterior face of the C, but also the edges oneither side. This may be done by causing the punch/die combination thatforms the leads 10 as shown in FIG. 3 to have a slightly larger thannormal tolerance, which creates a burr 30 at the edges of the lead 10.In so doing, the malleable solder curves around the lead 10 to cover thesides of the lead 10 as shown at 30 in FIG. 4. Depending on the actualtolerance of the punch/die combination, some of the sheet material 11may also be bent in a burr 32 toward the interior of the C-clip 12. Ascan be seen in FIG. 4, this may cause the main area of the C-clip 12 tobe held slightly away from the surface of the contact pad 18 of thesubstrate 50 to form a very narrow gap 34.

In assembly, a comb-like section of the carrier strip 14 with a desirednumber of leads 10 (for example, 10 to 50 in various applications) isengaged over the edge of a substrate 50. The leads 10 are spaced incorrespondence with the spacing of the contact pads 18 on the substrate50, which may be on either side or both sides of the substrate 50. EachC-clip 12 then resiliently engages a corresponding substrate contact pad18, and the leads 10 are thereby held in proper position with respect tothe substrate 50 for further operations.

After the substrate is engaged with the C-clip 12, as shown in FIG. 4,the clip/substrate assembly is moved through a heater, which melts thesolder layer 28. The liquid solder will migrate by capillary action intothe intervening gap 34 between the C-clip 12 and the contact point 18.The gap34 caused by a burr 32 on the lead edges improves the capillaryaction causing the solder to flow between the C-clip 12, and contactpoint 18 to improve the soldered bond. However, even without anysignificant burr 32, the solder will still sufficiently migrate into thespace between the C-clip 12 and the contact pad 18 because of inevitableirregularities between the C-clip 12 and the contact pad 18 when theyare engaged.

While the solder migrates, the top and bottom walls 22,24 of the C-clip12 or the burrs 32 will remain in contact with the contact point 18,retaining the clip in engagement with the substrate 50. On cooling, thesolder will re-solidify, forming a solid solder joint between the C-clip12 and the substrate 50. In this way, difficulties of relative movementofclip and substrate, which may be caused in the arrangement shown inthe prior Dennis U.S. Pat. No. 4,900,279, are avoided, and the lead 10is fixed to the substrate 50 at the desired point.

After joining the set of leads 10 to the substrate contact pads 18, thecarrier strip 14 is cut off. When this is done adjacent the point wherethe lead body joins the carrier strip, the end of the body 20 previouslynear the carrier 14 may be used as a terminal. This terminal portion maytake any desired configuration. It may, for example, be formed as a pintobe received in a connector or in an opening of a further printedcircuit board or the like, so as for example to be soldered thereto, oras a wire-wrap post. Alternatively, the portions of lead 10 beyond theC-clip (above it in FIG. 2) may be configured to be used as the terminalportion.

As an optional improvement, small apertures 52 may be formed in the lead10underneath the solder layer 28, which will facilitate flow of moltensolderinto the gap 34, by capillary action, without significantlyweakening the lead 10.

It will be understood that instead of forming a burr at the edge of theC-clip portion, that portion of the lead body may be bowed slightlyabove a longitudinal axis, to be slightly concave facing inwardly toprovide thespace 34 between the C-clip interior and the substrateconductive pad.

While the embodiment of the invention shown and described is fullycapable of achieving the results desired, it is to be understood thatthis embodiment has been shown and described for purposes ofillustration only and not for purposes of limitation. It will beapparent to those skilled in the art that variations and modificationscan be substituted herein without departing from the principles and truespirit of the invention, which is defined by the accompanying claims.

What is claimed is:
 1. A solderable lead for connection to a devicehaving a conductive area, comprising:a substantially flat elongatedbody, said body having a curved contact portion having an interior andan exterior, said interior being adapted to receive said device and toresiliently contact said conductive area of said device; and a layer ofsolder overlying said curved portion exterior and having a portionextending around the edge thereof and immediately adjacent said curvedportion interior, but without having solder in said interior of thecurved portion, whereby upon heating, said layer of solder will liquefyand migrate to form a soldered joint between said interior and saidconductive area.
 2. A solderable lead according to claim 1, wherein saidcurved contact portion is a C-shaped clip portion having top, rear andbottom walls.
 3. A solderable lead according to claim 2, wherein saidportion of said solder layer covers an edge of one of said top andbottom walls.
 4. A solderable lead according to claim 3, wherein portionof said clip portion under said solder layer is curved toward saidcurved portion interior.
 5. A solderable lead according to claim 3,wherein said C-shaped portion has a burr at the edge thereof, saidsolder layer extending over the edge of said burr.
 6. An assemblage ofleads, each as in claim 1, each lead being integral with a carrier stripand said leads being uniformly spaced along said carrier strip.
 7. Themethod of producing a lead soldered to a conductive pad of a substratecomprising the steps of:providing a lead as in claim 1; resilientlymaintaining said curved portion interior in contact with said substrateconductive area; heating said lead to melt said solder layer and causesaid solder to flow around the edge of said lead into the region betweensaid curved portion interior and said conductive area; whereby oncooling, said conductive pad is soldered to said curved portioninterior. .Iadd.
 8. A solderable lead for connection to a device havinga conductive area, comprising:a substantially flat elongated conductivebody having a body portion intended for soldering to said conductivearea, said body portion having a first surface facing said conductivearea and a second surface opposite said first surface; a plating-likesubstantially uniform thickness layer of solder formed from a band ofsolder overlying said body portion on said second surface, said solderlayer having a portion extending around the edge of said body portionand covering substantially the entire thickness of said edge, to beimmediately adjacent said first surface, but not covering said firstsurface; whereby upon heating said body in juxtaposition to saidconductive area said solder will liquify and migrate to form a solderedjoint between said body portion and said conductive area..Iaddend..Iadd.9. A solderable lead according to claim 8, wherein aportion of said body portion under said solder layer is curved towardsaid first surface. .Iaddend..Iadd.10. A solderable lead according toclaim 8, wherein said body portion has a burr at the edge thereof, saidsolder layer extending over the edge of said burr. .Iaddend..Iadd.11. Anassemblage of leads, each as in claim 8, each lead being integral with acarrier strip at one end of each lead and said leads being uniformlyspaced along said carrier strip. .Iaddend..Iadd.12. A method ofproducing a lead soldered to a conductive area of a substrate comprisingthe steps of:providing a lead having an elongated body with a bodyportion intended for soldering to said conductive area, said bodyportion having a first surface for facing said conductive area and asecond surface opposite said first surface, with a plating-like layer ofsolder of substantially uniform thickness overlying said body portion onsaid second surface and extending around the edges of said body portionbut short of said first surface; maintaining said first surface incontact with said substrate conductive area, heating said lead to meltsaid solder layer and cause said solder to flow around the edge of saidlead adjacent the region between said first surface and said conductivearea; whereby on cooling, said conductive area is soldered to said onesurface. .Iaddend..Iadd.13. A solderable lead for connection to a devicehaving a conductive area, comprising: an elongated body, said bodyhaving a substantially flat contact portion having a first surface and asecond surface, said first surface being adapted to contact saidconductive area of said device; and a plating-like substantially uniformthickness layer of solder overlying said second surface and having aportion extending around the edge thereof and immediately adjacent saidfirst surface, said solder layer overlying substantially the entirethickness of said edge, but without having solder on said first surface,whereby upon heating, said layer of solder will liquefy and migrate toform a solder joint between said first surface and said conductive area..Iaddend..Iadd.14. A method of producing solderable leads soldered toconductive areas of a substrate, comprising the steps of: providing asubstantially flat sheet of conductive material having a band of solderthereon, stamping from said sheet a series of leads, each lead having anelongated body with a body portion intended for soldering to saidconductive area, said body portion having a first surface for facingsaid conductive area and a second surface opposite said first surface,with a layer of solder overlying said body portion on said secondsurface, said stamping including causing solder from said band to extendaround the edge of the body portion of each lead. .Iaddend..Iadd.15. Amethod as in claim 14, wherein said step of stamping further comprisesforming a carrier section of said sheet joined to each lead at one endthereof. .Iaddend.